A NATION CHALLENGED: AVIATION; Use of Private Aircraft Remains Restricted

A month after terrorist attacks on the World Trade Center and the Pentagon, federal law enforcement officials are keeping a tight rein on private aviation, re-examining the ease with which aircraft can be purchased and flown.

While gun buyers must submit to criminal-background checks and go through a waiting period before purchasing a firearm, no such requirements exist for people seeking to buy a plane. Under Federal Aviation Administration regulations, would-be owners must simply show how they paid for or plan to finance the aircraft.

But in the wake of the attacks, the leaders of aviation trade groups have been contacted by the F.B.I., the Secret Service and the National Security Council about the risks to public safety posed by private planes.

The agencies' concern explains why the use of many of these aircraft remains restricted, particularly within 18-mile no-flight zones around New York and Washington, long after commercial jetliners have returned to the skies.

An F.A.A. spokesman, William Shumann, said, ''It's been clear from the beginning that decisions involving restrictions on aviation were not made by the F.A.A., but rather at higher levels of the government, including the National Security Council.''

This week, representatives of pilots and manufacturers of private planes pleaded for a meeting with Bush administration officials, arguing that the continued limits were curtailing their livelihoods and that law enforcement officials did not understand their plight.

''The security community is not particularly knowledgeable about the general aviation community,'' said John W. Olcott, president of the National Business Aviation Association, which represents nearly 7,000 companies that own or operate planes.

A White House official who refused to be identified by name cited ''security concerns'' as the reason for the continuing restrictions on private aviation. The situation remains under review, he said.

While leaders of trade groups say there is little cause for concern about private planes, some aviation experts favor careful review of existing rules and practices.

About 90 percent of the planes registered for private use in the United States are held in the names of corporations or the lenders that provided financing, making it impossible to immediately tell who actually owns them. There are about 250,000 or so planes in regular use, and some 70,000 change hands each year.

''This is one of the most vulnerable spots in all of aviation,'' said John Zimmerman, president of the Aviation Data Service, a research company based in Wichita, Kan. ''Nobody really knows who is in these planes.''

Worries about the threat posed by private aircraft are not new. During the gulf war, the F.A.A. looked into what might happen if a small plane crashed into United Nations headquarters or the White House, said Mary Schiavo, an aviation lawyer and author, who then served as inspector general of the Department of Transportation. No recommendations for action emerged from those reviews, she said.

Mr. Olcott said his association would support background checks for aircraft purchasers, should the government propose them.

For now, though, the aviation market is an abundant bazaar. Thousands of private planes are for sale over the Internet, from two-seat puddle jumpers costing a few thousand dollars to Boeing 747's, with prices running to $100 million or more.

The most expensive planes are sold primarily through unlicensed brokers who are not required to provide any information about their customers to authorities.

''Before Sept. 11, I'd show them the Airbus, take their money and run,'' said Richard E. Engles, president of one such business, Vance & Engles Aircraft Brokers, in Annapolis, Md. ''Now, I'd have to search deeply in my ethical soul,'' he said, before selling a plane to someone who aroused suspicions.

Fuel can cause the most damage in a plane crash. Engineers studying the collapse of the World Trade Center's twin towers attribute the buildings' destruction to heat generated by tons of burning jet fuel, not just the jetliners' impact.

Scott Miller, a professor of aviation engineering at Wichita State University in Kansas, estimates that a small Cessna 152, weighing 3,000 pounds and loaded with 26 gallons of fuel, would have the explosive force of a ton of TNT if it hit a building at 100 miles an hour. A 50,000-pound business jet, like those of Gulfstream Aerospace, flying at 300 miles an hour, would explode with the force of 141 tons of TNT.

With such calculations, some in private aviation emphasize caution. Charles Pierson, owner of the Transport Aircraft Corporation, an Indianapolis plane brokerage company, said he weeds out questionable customers by requiring a letter of intent with the potential owner's name. Mr. Pierson said his refusal to accept cash cost him a $235 million deal for several planes last year.

Ultimately, Ms. Schiavo said, ''the government has to deliver security and safety, because we have given them that job. We can't be sure what's flying over our heads anymore.''